A subdomain is the part of the web address that's before a domain name and you have most likely seen a lot of subdomains while exploring the Internet. As an example, many sites such as Wikipedia have versions in different languages using subdomains - en.wikipedia.org, de.wikipedia.org and so forth. The advantage of using a subdomain is that it can have an independent site and its own records, so you can even host it on another server. The practical use is that you can have a supplementary website, like an e-learning portal for college students as well as the primary school website. If you use subdomains as opposed to subfolders, it'll be much easier to perform maintenance or to upgrade a specific website, not mentioning that it will be more secure to have the websites separate from one another.